The water/isocyanate reaction also produces urea-groups at the end of the process. However, this reaction should not be considered to be a polyurea reaction as the mechanism is a two-step process, which is controlled by the much slower isocyanate water reaction and produces carbon dioxide.

Figure 2 / Applicability of the Different PU Chemistries

The Polyurethane Landscape

Hybrid systems already have a larger scope for application conditions, but the presence of catalysts in hybrids makes them more sensitive to humidity than "pure" polyurea systems. Moreover, because the catalyzed polyol/isocyanate reaction behaves differently from the amine/isocyanate reaction to changing application temperatures, the system becomes less robust.

Polyurea can be used in extreme conditions. When it is used on substrates almost saturated with water, polyurea will not provoke blistering nor will blistering occur when the air contains high amounts of humidity. Even at very low temperatures (as low as -20 deg C) the polyurea coating will still cure. Polyurea coatings combine high flexibility with hardness. They are the most suitable coatings when the following is required:

• high curing speed;
  
• application under high humidity and/or at low temperatures;
  
• extreme abrasion resistance;
  
• impermeable membranes;
  
• high thickness build up;
  
• chemical resistance.

Applications for Polyurea Coatings

The market development started in the United States, followed by Asia, with very strong growth during the second half of the 1990s. In the first stage of development, polyurea was used as a protective layer over polyurethane insulation foam for roofing applications. In Europe, the polyurea spray coatings market only started to develop in the last few years.

The broad window of application conditions, with a high tolerance for humidity, both from the environment and from the substrate and temperature, makes polyurea a very suitable coating for concrete in construction applications such as roof repair, containment liners, membranes, car park decks, bridges and offshore. The high abrasion resistance leads to its application in liners for truck, bulk transport wagons, freighters and conveyor belts. Table 2 presents an overview of the application fields where polyurea is chosen based on one or more of its unique application and/or film properties.

Raw Materials

Isocyanate

Standard polyurea spray coatings use MDI prepolymers with an NCO content of 15 to 16%. In this NCO range, a good compromise between viscosity of the material and the reactivity of the system is obtained. Lower-NCO prepolymers have a higher viscosity, but give higher elasticity and slower reactivity. Higher-NCO prepolymers are lower in viscosity, which provides an effective mixture of the two components. However, they become much more reactive, with the risk of building up more internal stress. Higher-NCO prepolymers will be used if higher surface hardness is needed. Table 3 provides an overview of the main properties of the MDI prepolymers used for polyurea spray coatings in Europe.

Diluent

Figure 3 / Carbamate Structure

• improved shelf life of the isocyanate-prepolymer;
  
• a compatibilizer for the mixing of the two components in the mixing chamber of the spray gun;
  
• a viscosity reducer for isocyanate-prepolymers;
  
• improved leveling of the applied film.

In applications where contact with water cannot be avoided, the use of propylene carbonate should also be limited, as propylene carbonate is completely miscible with water, and unreacted propylene carbonate could be extracted, increasing the water permeability of the film.

Huntsman Petrochemical Corp. owns certain patents relating to the use of propylene carbonate in polyurea elastomers. Other solvents or viscosity reducers can be used if they are compatible with the isocyanate component. They may be considered as a VOC. However, they will increase the shrinkage effect.

Polyetheramines

Chain Extenders

Other chain extenders like dimethylthio-toluenediamine (DMTDA), N,N'-di(sec.butyl)-amino-biphenyl methane (DBMDA) or 4,4'-methylenebis-(3-chloro, 2,6-diethyl)-aniline (MCDEA) slow down the reaction significantly. Table 5 lists various chain extenders and their characteristics. Significantly slowing down the reaction also means that the competition with the water reaction becomes more important and precautions need to be taken.

Additives, Fillers, Pigments

Product-Application Specifics

Due to the high cure speed of polyurea and the short mixing time, the products are mixed by impingement at high pressure. Indeed, for field applications it is preferable to formulate the products on a fixed 1:1 volume-mixing ratio. The pressure used in the field will vary between 150 and 250 bar. The viscosity of the products at application temperature ideally needs to be lower than 100 mPa.s, and the viscosity of the two components needs to be at the same level. The properties of these prepolymers can be found in Table 3. The viscosity of the resin blend at 25 deg C is approximately 900 mPa.s, dropping below 100 mPa.s at application temperature.

Experiments prove that polyurea films produced at 65 deg C, 70 deg C and 80 deg C have different properties, and these properties improve with increasing temperatures. The spraying equipment has improved significantly. The new spray equipment allows different temperature settings for the two components, ensuring an optimum mixing in the spray head. Other features are easier variable ratio settings; easy output control; and easy monitoring of application parameters.

The index of a polyurea system is typically kept at a slight over-index of the isocyanate in the range of 1.05-1.10. As the isocyanate group reacts to humidity, the excess isocyanate compensates for the 'loss' of isocyanate groups during storage and/or application. The film properties of the 1:1 volume ratio sprayed system were measured for an index variation between 0.90 and 1.15. The test results indicate that the film performs best at an index of 1.05 and higher. Below an index of 1.05 the results can vary significantly and become unpredictable, even for small index shifts.

Aspects of Spray Polyurea Technology

Initially, polyurea spray coatings looked too easy to apply. Polyurea is very fast - the coating can be put into service immediately after the application, and the final properties of the coating are obtained only a few hours afterwards. Polyurea is not water- or temperature-sensitive, and is easy to formulate and produce. The first systems on the market were indeed very fast, with a gel time of less than two seconds, and initially a number of problems were linked to the reactivity of the systems.

At first, substrate wetting was a problem. This problem was linked to the development phase of polyurea with the use of extremely fast spray systems. Development programs focusing on adhesion on concrete, with polyurea systems presenting gel times of three to four seconds, resulted in cohesive adhesion failure in the concrete. In practice, to limit the risks under variable field conditions, a multi-layer system is applied, made of a primer and a topcoat.

A second problem noted in the field was the lack of intercoat adhesion. Lab tests, with times between coats of several weeks, have shown that intercoat adhesion is very good. When problems occur with intercoat adhesion, most of the time they can be related back to problems with the raw materials, the manufacturing of the systems or the spray equipment. Spray equipment problems, or a disturbance of the feeding of one or both components towards the mixing module, can cause poor mixing. Adapting the machine settings of the spray can solve this.

Due to the high reactivity of the systems, the surface quality of the sprayed film was initially very poor. Fine-tuning the spraying equipment was a first step towards solving the problem. The use of non-VOC reactive diluents and the development of new MDI prepolymers with higher 2,4'-isomer content resulted in perfect surface quality without compromising on working time.

The cost of polyurea spray coatings technology is seen as a barrier to entry. "Pure" polyurea systems are more expensive, when considering raw materials cost alone, but can be applied in areas where all other systems will fail or where they are not suitable. Also the initial investment in equipment is rather costly.

However, when estimating the capital cost for a project, polyurea is more competitive when both the processing time and the waiting period before the coated substrate is put back into service are included.

As discussed above, the success of the project is very equipment- and applicator-dependent, and we believe that the high entry barrier can only guarantee quality services from specialized and skilled operators.

Construction-Related Aspects

If primers are considered, a good approach is to start with the evaluation of existing primers with known performance on the substrate. It is important to determine the adhesion performance of the polyurea coating on the primers and to check whether the application conditions of the primer and the re-coating conditions for the primer still work for a system with polyurea as a finish.

Concrete Surface Defects and Surface Preparation

The preparation of the surface is extremely important. Depending on the surface quality of the concrete, one or more of the following actions needs to be taken:

• water jet and/or solvent cleaning;
  
• grit blasting;
  
• bughole and crack filling;
  
• repair layer of concrete;
  
• priming.

Steel Surface Defects and Surface Preparation

• molecular attraction of the interfacial forces from both the coating and the substrate,

• mechanical bonding or anchoring of the coating on the substrate.

On freshly grit-blasted, dust-free steel with a surface roughness SA 2 1/2 to SA 2 according to the specification ISO 8501-1, very high adhesion values can be obtained for polyurea, even without the use of primers.

Figure 4 / Mixing Efficiency Influence on the Physical Properties of a Polyurea Coating

Mixing Efficiency

Figure 5 / Physical Property Variations at Varying System Index

System Index Influence on Final Film Properties

Taking into account that, in practice, slight variations might occur in the application parameters, related to the precision of the spray equipment and variations in application conditions, it is safer to work at a minimum index of 1.10 to 1.15.

Figure 6 / Physical Property Development as a Function of Filler Content

Filler Influence on Physical Properties

Depending on the performance of the spray installation, the dosing of filler can vary. We added up to 40% of filler to the resin blend. The main limiting factor for the processing is the increase in viscosity of the filled component. This can result in difficulties with the pumping unit and/or poor mixing due to big differences in viscosity for both components.

Adding fillers improves the surface hardness, the angle tear and flexural bending modulus but has a negative influence on the elongation and the tear propagation or trouser tear (Figure 6).

Figure 7 / Water Absorption for Different SUPRASEC Prepolymers

Water Absorption

As can be seen in Figure 7, the unmodified standard polyurea spray coatings give only a limited protection against corrosion. The water absorption drops significantly with increasing the NCO-content for the prepolymer. Further fine-tuning of the pre-polymer resulted in water absorption being well below 0.5% after 10 days. Cathodic disbondment testing on this system gave very satisfying test results.

Anti-Skid Performance

One example is the outdoor, impact-absorbing playground flooring. The system tested is based on SUPRASEC 2049, formulated to be applied in a 1 to 1 volume ratio. The film properties are 70 Shore A hardness, 600% elongation and 13 N/mm2 tensile strength. The results in Table 6 show that it is perfectly possible to formulate a system, which complies with the antiskid needs of a flooring system, in this case for a flexible substrate.

Conclusions

Polyureas are very suitable for construction applications. The fast curing makes it possible to use when only very short disturbance periods are allowed. The fact that the isocyanate/water reaction is not affecting the physical properties of the applied film, expands the use of polyurea to high relative humidity conditions and does not set such stringent limits on the water content of substrates like concrete. Although they slow down at colder temperatures, polyurea coatings still cure at temperatures where other chemistries fail.

The formulation of polyurea spray coatings has to be approached similarly to any other coating system. Careful selection of raw materials for fine-tuning of the formulation and evaluation of the system, in the sometimes difficult conditions where the coating is to be applied, is still necessary.

Polyurea spray coating technology means handling reactive chemicals. While handling the chemicals during their manufacturing, packaging and application, the correct protective clothing should be worn at all times.

Acknowledgements

For more information, e-mail Marc Broekaert at marc_broekaert@huntsman.com.

References

SUPRASEC is a registered trademark of Huntsman International LLC. JEFFAMINE and JEFFSOL are registered trademarks of Huntsman Petrochemicals Corp.

This paper was presented at the Fourth European Congress on Construction Chemicals on April 10, 2003 in Nurnberg, Germany (parallel to the European Coatings Show 2003).